Succinyl-CoA synthetase (SCS) catalyzes the substrate-level phosphorylation step of the citric acid cycle. In ketone body metabolism, SCS generates succinyl-CoA from succinate, CoA and ATP; and CoA transferase (CoAT) uses this succinyl-CoA in the activation of the ketone body acetoacetate, producing succinate and acetoacetyl-CoA. We have determined the structure of E. coli SCS using 23 crystals. We intend to collect data to the diffraction limit of these crystals using one crystal and cryotemperatures. These crystals grow in the presence of CoA and the structure determination demonstrated the CoA-binding site. The binding site for the nucleotide was detem-iined using data from crystals soaked with ADP, proving that the histidine residue phosphorylated in the reaction must swing -30 A between the binding site for CoA and the binding site for the nucleotide. To interpret the catalytic mechanism, the binding site of succinate or the succinyl moiety must be known. Crystals of pig heart SCS grow in the absence of CoA and we have successfully soaked them with CoA to determine the structure of the complex. We propose collecting data from crystals soaked with a non-hydrolyzable analogue of succinyl-CoA to determine the succinyl binding site. The structure of CoA transferase is yet to be determined because attempts at soaking the monoclinic crystals with heavy atoms have led to noniso-morphism. Data collection was on BioCARS Station 14 BM-C.